Methods for preparing polyvinylidene fluoride composites

ABSTRACT

An electrically conductive composite comprising a polyvinylidene fluoride polymer or copolymer and carbon nanotubes is provided. Preferably, carbon nanotubes may be present in the range of about 0.5-20% by weight of the composite.  
     The composites are prepared by mixing or dispersing carbon nanotubes in polymer emulsion using an energy source such as a Waring blender. The liquid in the mixture is then evaporated to obtain the composite comprising the polymer and the nanotubes.

RELATED APPLICATIONS

[0001] This application is a continuation in part of U.S. Ser. No.09/903,189, filed Jul. 11, 2001.

FIELD OF THE INVENTION

[0002] The invention relates generally to electrically conductivepolyvinylidene fluoride composites containing carbon nanotubes, and themethods for preparing them.

BACKGROUND OF THE INVENTION

[0003] Polyvinylidene Fluoride Plastics are synthetic polymers whichhave a wide range of properties that make them useful for a variety ofapplications ranging from packaging and building/construction totransportation; consumer and institutional products; furniture andfurnishings; adhesives, inks and coatings and others. In general,plastics are valued for their toughness, durability, ease of fabricationinto complex shapes and their electrical insulation qualities.

[0004] One such widely used plastic is polyvinylidene fluoride(—H₂C═CF₂—), (“PVDF”), which is the homopolymer of 1,1-difluoroethylene,and is available in molecular weights between 60,000 and 534,000. Thisstructure, which contains alternating —CH₂— and —CF₂— groups along thepolymer backbone, gives the PVDF material polarity that contributes toits unusual chemical and insulation properties.

[0005] PVDF is a semicrystalline engineered thermoplastic whose benefitsinclude chemical and thermal stability along with melt processibilityand selective solubility. PVDF offers low permeability to gases andliquids, low flame and smoke characteristics, abrasion resistance,weathering resistance, as well as resistance to creep and otherbeneficial characteristics. As a result of its attractive properties,PVDF is a common item of commerce and has a wide variety of applications(e.g., cable jacketing, insulation for wires and in chemical tanks andother equipments).

[0006] In addition to forming a homopolymer, PVDF also form co-polymerswith other polymer and monomer families, most commonly with theco-monomers hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE),and tetrafluoroethylene (TFE), as well as terpolymers and olefins. Theproperties of the copolymers is strongly dependent on the type andfraction of the co-monomers as well as the method of polymerization. Forexample, HFP makes a homogenous copolymer with PVDF. On the other hand,the PVDF copolymer phase segregates if the other monomer is notfluorinated.

[0007] Conductive Plastics

[0008] Recently, demand and applications for electrically conductiveplastics have grown. In these uses, one seeks to exploit the uniqueproperties of plastics, often as an alternative to metals. For example,electrically conductive polymeric materials are desirable for manyapplications including the dissipation of electrostatic charge fromelectrical parts, electrostatic spray painting and the shielding ofelectrical components to prevent transmission of electromagnetic waves.

[0009] Conductivity (i.e., the ability of material to conduct ortransmit heat or electricity) in plastics is typically measured in termsof bulk resistivity (i.e., volume resistivity). Bulk resistivity, whichis the inverse of conductivity, is defined as the electrical resistanceper unit length of a substance with uniform cross section as measured inohm-cm. Thus, in this manner, the electrical conductivity of a substanceis determined by measuring the electrical resistance of the substance.

[0010] Electrically conductive plastics can be divided into severalcategories according to their use. For example, high level ofresistivity (i.e., low level of conductivity) ranging from approximately10⁴ to 10⁸ ohm/cm generally confer protection against electrostaticdischarge (“ESD”) and is referred to as the ESD shielding level ofconductivity. This is also the level of conductivity needed forelectrostatic painting. The next level of resistivity, which ranges fromapproximately 10⁴ ohm/cm and lower, protects components contained withinsuch plastic against electromagnetic interference (“EMI”) as well asprevents the emission of interfering radiation, and is referred to asthe EMI shielding level of conductivity. In order for a plastic articleto be used as a conductive element like a current collector or separatorplate in an electrochemical cell, resistivity less than 10² ohm/cm isrequired.

[0011] The primary method of increasing the electrical conductivity ofplastics have been to fill them with conductive additives such asmetallic powders, metallic fibers, ionic conductive polymers,intrinsically conductive polymeric powder, e.g., polypyrrole, carbonfibers or carbon black. However, each of these approaches has someshortcomings. Metallic fiber and powder enhanced plastics have poorcorrosion resistance and insufficient mechanical strength. Further,their density makes high weight loadings necessary. Thus, their use isfrequently impractical.

[0012] When polyacrylonitrile (“PAN”) or pitch-based carbon fiber isadded to create conductive polymers, the high filler content necessaryto achieve conductivity results in the deterioration of thecharacteristics specific to the original resin. If a final product witha complicated shape is formed by injection molding, uneven fillerdistribution and fiber orientation tends to occur due to the relativelylarge size of the fibers, which results in non-uniform electricalconductivity.

[0013] Principally because of these factors and cost, carbon black hasbecome the additive of choice for many applications. The use of carbonblack, however, also has a number of significant drawbacks. First, thequantities of carbon black needed to achieve electrical conductivity inthe polymer or plastic are relatively high, i.e. 10-60%. Theserelatively high loadings lead to degradation in the mechanicalproperties of the polymers. Specifically, low temperature impactresistance (i.e., a measure of toughness) is often compromised,especially in thermoplastics. Barrier properties also suffer. Sloughingof carbon from the surface of the materials is often experienced. Thisis particularly undesirable in many electronic applications. Similarly,outgassing during heating may be observed. This adversely affects thesurface finish. Even in the absence of outgassing, high loadings ofcarbon black may render the surface of conductive plastic partsunsuitable for automotive use.

[0014] Taken as a whole, these drawbacks limit carbon black filledconductive polymers to the low end of the conductivity spectrum. For EMIshielding or higher levels of conductivity, the designer generallyresorts to metallic fillers with all their attendant shortcomings or tometal construction or even machined graphite.

[0015] What ultimately limits the amount of carbon black that can be putinto plastic is the ability to form the part for which the plastic isdesired for. Depending on the plastic, the carbon black, and thespecific part for which the plastic is being made, it becomes impossibleto form a plastic article with 20-60 wt % carbon black, even if thephysical properties are not critical.

[0016] Carbon Fibrils

[0017] Carbon fibrils have been used in place of carbon black in anumber of polymer applications. Carbon fibrils, referred toalternatively as nanotubes, whiskers, buckytubes, etc., are vermicularcarbon deposits having diameters less than 1.0μ and usually less than0.2μ. They exist in a variety of forms and have been prepared throughthe catalytic decomposition of various carbon-containing gases at metalsurfaces. Such fibers provide significant surface area when incorporatedinto a structure because of their size and shape. They can be made withhigh purity and uniformity.

[0018] It has been recognized that the addition of carbon fibrils topolymers in quantities less than that of carbon black can be used toproduce conductive end products. For example, U.S. Pat. No. 5,445,327,hereby incorporated by reference, to Creehan disclosed a process forpreparing composites by introducing matrix material, such asthermoplastic resins, and one or more fillers, such as carbon fibers orcarbon fibrils, into a stirred ball mill. Additionally, U.S. Ser. No.08/420,330, entitled “Fibril-Filled Elastomer Compositions,” alsoincorporated by reference, disclosed composites comprising carbonfibrils and an elastomeric matrix, and methods of preparing such.

[0019] It has also been recognized that the addition of carbon fibrilsto polymers can be used to enhance the tensile and flexuralcharacteristics of end products. (See, e.g. Goto et al., U.S.Application Ser. No. 511,780, filed Apr. 18, 1990, and herebyincorporated by reference.) Additionally, prior work by Moy et al., U.S.application Ser. No. 855,122, filed Mar. 18, 1992, and Uehara et al.,U.S. application Ser. No. 654,507, filed Feb. 23, 1991, bothincorporated by reference, disclosed the production of fibril aggregatesand their usage in creating conductive polymers. Moy et al. disclosedthe production of a specific type of carbon fibril aggregate, i.e.combed yarn, and alluded to its use in composites. Uehara et al. alsodisclosed the use of fibril aggregates in polymeric materials. Thefibril aggregates have a preferred diameter range of 100-250 microns.When these fibril aggregates are added to polymeric compositions andprocessed, conductivity is achieved.

[0020] U.S. Pat. No. 5,643,502 to Nahass et al., hereby incorporated byreference, disclosed that a polymeric composition comprising a polymericbinder and 0.25-50 weight % carbon fibrils had significantly increasedIZOD notched impact strength (i.e., greater than about 2 ft-lbs./in) anddecreased volume resistivity (i.e., less than about 1×10¹¹ ohm-cm).Nahass disclosed a long list of polymers (including polyvinylidenefluoride) into which carbon fibrils may be dispersed to form acomposite. The polymers used by Nahass in the Examples of the '502patent for preparing conductive, high toughness polymeric compositionsinclude polyamide, polycarbonate, acrylonitrile-butadiene-styrene, poly(phenylene ether), and thermoplastic urethane resins and blends.

[0021] While the nanotube-containing polymer composites of the art areuseful and have valuable strength and conductivity properties, many newuses for such composites require that very high strength and lowconductivity be achieved with low nanotube loading in the polymer.Accordingly, the art has sought new composite compositions which achievethese ends.

OBJECTS OF THE INVENTION

[0022] It is a primary object of the invention to provide a polymercomposite which is mechanically strong and electrically conductive.

[0023] It is a particular object of the invention to provide a polymercomposite which has a higher level of conductivity than known polymercomposites.

[0024] It is yet another object of the invention to provide polymercomposites which achieve extraordinary levels of conductivity at lowlevels of nanotube loading.

[0025] It is a further object of the invention to provide methods forpreparing a polymer composite which is mechanically strong andelectrically conductive.

SUMMARY OF THE INVENTION

[0026] It has now been discovered that composites containingpolyvinylidene fluoride polymer or copolymer and carbon nanotubes haveextraordinary electrical conductivity. Composites with less than 1% byweight of carbon nanotubes have been found to have a bulk resistivitymany times lower than the bulk resistivity of other polymer compositeshaving similar nanotube loading. Composites with as little as 13% byweight carbon nanotubes have a bulk resistivity similar to that of purecarbon nanotube mats.

[0027] Composites containing polyvinylidene fluoride polymer orcopolymer and carbon nanotubes may be prepared by dissolving the polymerin a solvent to form a polymer solution and then adding the carbonnanotubes into the solution. The solution is mixed using a sonicator ora Waring blender. A precipitating component is added to precipitate outa composite comprising the polymer and the nanotubes. The composite isisolated by filtering the solution and drying the composite.

[0028] Composites containing polyvinylidene fluoride polymer orcopolymer and carbon nanotubes may also be prepared by adding carbonnanotubes to a polymer emulsion and then mixing the emulsion with aWaring Blender. The water/solvent is then removed by evaporation toobtain the composite comprising the polymer and the nanotubes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a graph plotting composite resistivity as a function ofnanotube loading in a PVDF/HFP composite.

[0030]FIG. 2 is a graph plotting composite resistivity as a function ofgraphite concentration for PVDF composites with 13% nanotube loading.

[0031]FIG. 3 is a logarithmic graph plotting resistivity as a functionof nanotube weight fraction for various polymer composites.

[0032]FIG. 4 is a graph plotting composite conductivity as a function ofnanotube loading in various polymer composites.

DETAILED DESCRIPTION OF THE INVENTION

[0033] PVDF-Nanotube Composites

[0034] It has now been discovered that composites containing PVDFpolymer or copolymer and carbon nanotubes have electrical conductivitiesmuch higher than other polymer/carbon nanotube composites known in theart. As used hereafter, the term “PVDF composite” refers broadly to anycomposite containing PVDF or a copolymer of vinylidene fluoride andanother monomer, and carbon nanotubes. Unlike other polymer composites,PVDF composites with bulk resistivities as low as pure carbon nanotubescan be formed. For simplicity, the term percent nanotube loading (%nanotube loading) will be used to refer to percentage of nanotube byweight in the composite.

[0035] It has now been discovered that lower loadings of nanotubes inPVDF composites results in far higher conductivities than similarloadings in other polymer composites. For example, a PVDF composite with5% nanotube loading has a bulk resistivity of 0.42 ohm-cm while apoly(paraphenylene sulfide) composite with 5% nanotube loading has abulk resistivity of 3.12 ohm-cm.

[0036] PVDF composites containing carbon nanotubes in an amount aslittle as 1% or less by weight have an exceptionally low bulkresistivity compared to the pure PVDF polymer or copolymer, and haveexceptionally low resistivity compared to other polymer composites atsimilar nanotube loadings. Nanotube loading may be widely varied. Forexample, PVDF composites may be made with nanotube loadings of broadlyfrom 0.01-30% desirably from 0.5-20% and preferably from 1-15%. It hasbeen found that PVDF composites have much lower bulk resistivitiescompared to other polymer composites at any given nanotube loading.

[0037] It has been further discovered that a PVDF composite with aslittle as about 13% nanotube loading has a bulk resistivity comparableto a pure nanotube mat, i.e., between 0.02 ohm-cm to 0.08 ohm-cm. PVDFcomposites with about 13% to 20% nanotube loading all have bulkresistivity values within the range of the resistivity values of a purenanotube mat. PVDF composites can be formed with bulk resistivity ofless than about 10 ohm-cm or less than about 1 ohm-cm. The bulkresistivity of PVDF composite may be adjusted by varying the nanotubeloading to meet the level of conductivity required for its intendedapplication.

[0038] Depending on how the composite is prepared, no furtherimprovements in conductivity beyond 13-20% nanotube loading in PVDF wereobserved, the limiting resistivity of 0.02 ohm-cm (i.e., the resistivityof a pure mat of carbon nanotubes) having been reached. The lower limitof nanotube loading is set by the limit of percolation and will dependon various factors such as the method of composite formation, thematerials used, etc. For example, Table 1 shows that the lower limit ofnanotube loading under the conditions in Example 1 is well under 1%, butapparently above 0.2%.

[0039] The monomers which may be used with vinylidene fluoride monomerto form PVDF copolymers for the composites of the invention includehexafluoropropylene, polystyrene, polypropylene, CTFE, TFE, terpolymersor olefins. The copolymers may be produced broadly from a de minimsamount of a monomer other than vinylidene fluoride to as much as 90% byweight of such monomer. Desirably copolymers of the invention containfrom 1% to 70% by weight of such other monomer and preferably from 10%to 50% by weight thereof.

[0040] The PVDF composites of the invention also include mixtures ofPVDF and other polymers, including those wherein the PVDF and otherpolymers are miscible or immiscible with one another. The PVDFcomposites of the invention also include mixtures of PVDF and copolymersformed from vinylidene fluoride and another monomer, as described above,and mixtures of these mixtures with other polymers.

[0041] Fillers such as graphite may also be used with PVDF copolymercomposites.

[0042] Carbon Nanotubes

[0043] A variety of different carbon nanotubes may be combined with PVDFor PVDF copolymers to form the composites of the present invention.Preferably, the nanotubes used in the invention have a diameter lessthan 0.1 and preferably less than 0.05 micron.

[0044] U.S. Pat. No. 4,663,230 to Tennent, hereby incorporated byreference, describes carbon fibrils that are free of a continuousthermal carbon overcoat and have multiple ordered graphitic outer layersthat are substantially parallel to the fibril axis. U.S. Pat. No.5,171,560 to Tennent et al., hereby incorporated by reference, describescarbon nanotubes free of thermal overcoat and having graphitic layerssubstantially parallel to the fibril axes such that the projection ofsaid layers on said fibril axes extends for a distance of at least twofibril diameters. As such, these Tennent fibrin may be characterized ashaving their c-axes, the axes which are perpendicular to the tangents ofthe curved layers of graphite, substantially perpendicular to theircylindrical axes. They generally have diameters no greater than 0.1μ andlength to diameter ratios of at least 5. Desirably they aresubstantially free of a continuous thermal carbon overcoat, i.e.,pyrolytically deposited carbon resulting from thermal cracking of thegas feed used to prepare them. These fibrils are useful in the presentinvention. These Tennent inventions provided access to smaller diameterfibrils having an ordered outer region of catalytically grown multiple,substantially continuous layers of ordered carbon atoms having anoutside diameter between about 3.5 to 70 nm, and a distinct inner coreregion, each of the layers and the core being disposed substantiallyconcentrically about the cylindrical axis of the fibrils, said fibrilsbeing substantially free of pyrolytically deposited thermal carbon.Fibrillar carbons of less perfect structure, but also without apyrolytic carbon outer layer have also been grown.

[0045] Geus, U.S. Pat. No. 4,855,091, hereby incorporated by reference,provides a procedure for preparation of fishbone fibrils substantiallyfree of a pyrolytic overcoat. When the projection of the graphiticlayers on the nanotube axis extends for a distance of less than twonanotube diameters, the carbon planes of the graphitic nanotube, incross section, take on a herring bone appearance. Hence, the termfishbone fibrils. These carbon nanotubes are also useful in the practiceof the invention.

[0046] The “unbonded” precursor nanotubes may be in the form of discretenanotubes, aggregates of nanotubes, or both.

[0047] Nanotubes aggregate in several stages or degrees. Catalyticallygrown nanotubes produced according to U.S. Ser. No. 08/856,657, filedMay 15, 1997, hereby incorporated by reference, are formed in aggregatessubstantially all of which will pass through a 700 micron sieve. About50% by weight of the aggregates pass through a 300 micron sieve. Thesize of as-made aggregates can, of course, be reduced by various means,but such disaggregation becomes increasingly difficult as the aggregatesget smaller.

[0048] Nanotubes may also be prepared as aggregates having variousmorphologies (as determined by scanning electron microscopy) in whichthey are randomly entangled with each other to form entangled balls ofnanotubes resembling bird nests (“BN”); or as aggregates consisting ofbundles of straight to slightly bent or kinked carbon nanotubes havingsubstantially the same relative orientation, and having the appearanceof combed yarn (“CY”) e.g., the longitudinal axis of each nanotube(despite individual bends or kinks) extends in the same direction asthat of the surrounding nanotubes in the bundles; or, as aggregatesconsisting of straight to slightly bent or kinked nanotubes which areloosely entangled with each other to form an “open net” (“ON”)structure. In open net structures, the extent of nanotube entanglementis greater than observed in the combed yarn aggregates (in which theindividual nanotubes have substantially the same relative orientation)but less than that of bird nest. CY and ON aggregates are more readilydispersed than BN making them useful in composite fabrication whereuniform properties throughout the structure are desired.

[0049] The morphology of the aggregate is controlled by the choice ofcatalyst support. Spherical supports grow nanotubes in all directionsleading to the formation of bird nest aggregates. Combed yarn and opennest aggregates are prepared using supports having one or more readilycleavable planar surfaces, e.g., an iron or iron-containing metalcatalyst particle deposited on a support material having one or morereadily cleavable surfaces and a surface area of at least 1 squaremeters per gram. Moy et al., U.S. application Ser. No. 08/469,430entitled “Improved Methods and Catalysts for the Manufacture of CarbonFibrils”, filed Jun. 6, 1995, hereby incorporated by reference,describes nanotubes prepared as aggregates having various morphologies.

[0050] Further details regarding the formation of carbon nanotube ornanofiber aggregates may be found in U.S. Pat. No. 5,165,909 to Tennent;U.S. Pat. No. 5,456,897 to Moy et al.; Snyder et al., U.S. patentapplication Serial No. 149,573, filed Jan. 28, 1988, and PCT ApplicationNo. US89/00322, filed Jan. 28, 1989 (“Carbon Fibrils”) WO 89/07163, andMoy et al., U.S. patent application Ser. No. 413,837 filed Sep. 28, 1989and PCT Application No. US90/05498, filed Sep. 27, 1990 (“FibrilAggregates and Method of Making Same”) WO 91/05089, and U.S. applicationSer. No. 08/479,864 to Mandeville et al., filed Jun. 7, 1995 and U.S.application Ser. No. 08/329,774 by Bening et al., filed Oct. 27, 1984and U.S. application No. 08/284,917, filed Aug. 2, 1994 and U.S.application Ser. No. 07/320,564, filed Oct. 11, 1994 by Moy et al., allof which are incorporated by reference.

[0051] Other fibrils of different microscopic and macroscopicmorphologies useful in the present invention include the multiwalledfibrils disclosed in U.S. Pat. Nos. 5,550,200, 5,578,543, 5,589,152,5,650,370, 5,691,054, 5,707,916, 5,726,116, and 5,877,110 each of whichare incorporated by reference.

[0052] Single walled fibrils may also be used in the composites of theinvention. Single walled fibrils and methods for making them aredescribed in U.S. Pat. No. 6,221,330 and WO 00/26138, both of which arehereby incorporated by reference. Single walled fibrils havecharacteristics similar to or better than the multi-walled fibrilsdescribed above, except that they only have a single graphitic outerlayer, the layer being substantially parallel to the fibril axis.

[0053] PVDF composites containing carbon nanotubes with differentgrades, sizes, morphologies, or types have different bulk resistivity ata given fibrinl loading. For example, it has been found that combedcandy (“CC”) nanotubes provide lower bulk resistivity than bird nest(“BN”) nanotubes in PVDF composites at low nanotube loading. Withoutwishing to be bound by any theory, it is believed that CC nanotubes,which are aggregated in parallel bundles, are easier to disperse in thepolymer than BN nanotubes, resulting in a more even distribution offibrils in the composite and hence, lower bulk resistivity.

[0054] The conductivity levels obtained using PVDF composites formedfrom PVDF polymer or copolymers and carbon nanotubes make it possible touse conductive plastic, with all its property and fabricationadvantages, in place of metals or pure graphite in a number ofapplications.

[0055] Use of PVDF Composites

[0056] PVDF composites of the invention may be used in applicationswhere exceptional electrical conductivity is important. Examples of suchuses include current collectors for high power electrochemicalcapacitors and batteries. Current commercial materials used for thesepurposes have bulk resistivities of approximately 1 ohm-cm. Otherapplications include conducting gaskets or EMF shield coatings. In theseapplications, a difference of, for example, 0.04 ohm-cm in bulkresistivity will have a very significant impact on product performance.

[0057] Still further uses include bipolar plates for PEM fuel cells aswell as bifunctional (binder and conductivity enhancers) additives to alithium battery cathode. These bipolar plates are formed by preparing aPVDF composite as disclosed herein and then extruding a PVDF compositesheet with a thickness of, for example, 2 mm. A single screw extrudermay be used for the sheet extrusion. Flow channels may be engravedbetween two hot plates, one with a mirror pattern of the front platechannel and the other with a mirror pattern of the back plate channel.The channels may run parallel to each other from one corner to another,with each channel separated from the other by 0.5 mm. The channels mayhave a width and depth of 0.5 mm.

[0058] Method of Preparing Composites

[0059] PVDF composites may be prepared by a solution method in whichPVDF polymer or copolymer is dissolved in a solvent such as acetone toform a solution. Other soluble solvents such as tetrahydrofuran, methylethyl ketone, dimethyl formamide, dimethyl acetamide, tetramethyl urea,dimethyl sulfoxide, trimethyl phosphate, 2-pyrrolidone, butyrolacetone,isophorone, and carbitor acetate may be used.

[0060] Nanotubes are dispersed in the solvent by applying energy to thepolymer-nanotube mixture. The energy source can be a mechanicalhomogenizer, ultrasonic sonifier, high speed mixer, Waring blender, orany other mixing means known in the art. A precipitating component suchas water is added to precipitate or quench the solid compositecontaining the polymer and the nanotubes. The precipitating componentmay be any medium which is miscible with the solvent, but in which thePVDF polymer or copolymer mixture is insoluble.

[0061] The solvent may optionally be removed by filtration orevaporation and dried to isolate the PVDF composite. The composite maybe isolated by drying or evaporating steps such as heat drying, vacuumdrying, freeze-drying, etc. known in the art.

[0062] PVDF composites may also be prepared by a melt compoundingprocess in which the PVDF polymer or copolymer is mixed with nanotubesin the mixing head of a mixer such as a Brabender mixture at hightemperatures (i.e., over 200° C.) to melt and compound the PVDF polymeror copolymer into the carbon nanotubes to form the composite.

[0063] Once the composite has been obtained, it may then be molded asnecessary using compression or injection molding equipment and methodsknown in the art.

[0064] PVDF composites prepared using the solvent solution method havesignificantly lower bulk resistivity and thus were better electricalconductors, than PVDF composites made using traditional melt compoundingmethods. Without wishing to be bound by any theory, it is believed thatthe solvent solution method allows for better intermixing of the PVDFwith the carbon nanotubes in the PVDF composites, thus resulting inlower bulk resistivity.

[0065] It has also been found that at low nanotube loadings, usingsonicators or ultrasonic sonifiers resulted in PVDF composites havinglower bulk resistivities than PVDF composites made using mechanicalmixing means such as Waring blenders. Without wishing to be bound by anytheory, it is also believed that sonicators or ultrasonic sonifiers areable to better disperse low levels of nanotubes in the polymer thanmechanical mixing means, resulting in better distribution of nanotubeswithin the composite and hence, better conductivity.

[0066] For any industrial processes where use of organic solvents is notpreferred, PVDF composites may be prepared using PVDF emulsions. In thismethod, carbon nanotubes are directly mixed with or dispersed in PVDFemulsions (or PVDF latex, or any dispersions of PVDF in water) byapplying an energy source such as a mechanical homogenizer, ultrasonicsonifier, high speed mixer, Waring blender, or any other mixing meansknown in the art. The water (or liquid) in the mixture is then removed,for example, by evaporation or any drying means known in the art torecover the PVDF composite.

EXAMPLES

[0067] Examples of electrically conductive PVDF composites and methodsof preparing the same are set forth below.

Example I

[0068] A PVDF polymer, Kynar 761, was obtained from Elf Atochem anddissolved in acetone. Hyperion CC carbon nanotubes were added anddispersed into the polymer solution for two to five minutes using a highshear blender (i.e., Waring blender). Water was added to the dispersionto precipitate out the polymer with the nanotubes. The material wasfiltered and the filtrate was dried in a vacuum oven at 100° C. toremove acetone and water, leaving behind the dry nanotube/PVDFcomposite. Multiple sheets of the composite with thickness of 0.003-0.01inches were made using a compression molder. Bulk resistivity of thethin sheet samples was measured using a four probe method. Tensilestrength was also measured. Multiple batches with different amounts ofcarbon nanotubes were tested. The results are reported in Table 1 below:TABLE 1 Nano- Nano- Thick- Tensile tubes PVDF tubes ness StrengthResistivity Batch # (g) (g) (%) (inch) (psi) (Ohm-cm) 1 .02 10 0.20 — —300,000 2 .09 10 0.89 0.0075 7005 73.7404 0.0107 7005 64.5745 0.00357005 88.5977 0.0109 7005 87.3533 3 .11 10 1.09 0.0102 8145 22.55720.0058 8145 15.7831 4 .31 10 3.01 0.0090 8072 1.2106 0.0099 8072 1.21890.0075 8072 1.3074 0.0067 8072 1.2219 5 .53 10 5.03 0.0055 6739 0.39240.0080 6739 0.6255 0.0100 6739 0.4890 0.0050 6739 0.3912 6 .61 8 7.080.0030 6770 0.2934 0.0040 6770 0.2992 0.0030 6770 0.2554 0.0098 67700.2819 7 1.5 15 9.09 0.0078 8025 0.2154 0.0100 8025 0.2359 0.0081 80250.2470 0.0090 8025 0.2175 8 1.24 10 11.03 0.0115 7144 0.1336 0.0132 71440.1177 0.0142 7144 0.1152 0.0130 7144 0.1496 9 1.2 8 13.04 0.0115 75210.1012 0.0125 7521 0.0811 0.0108 7521 0.0938 0.0081 7521 0.0769 10 1.5 620.00 0.0120 5318 0.0387 0.0110 5318 0.0443 0.0065 5318 0.0441 0.00755318 0.0381 0.0130 5318 0.0419 11 2 6 25.00 0.0097 1918 0.0391 0.00901918 0.0371 0.0090 1918 0.0497 0.0120 1918 0.0483

[0069] The results of Example I show that a PVDF composite with lessthan 1% nanotube loading had a significantly lower bulk resistivity thana pure PVDF polymer. The bulk resistivity of the composites droppedsignificantly as the nanotube loading increased to approximately 3%. Atapproximately 5% nanotube loading, the bulk resistivity of the PVDFcomposite was below 1 ohm-cm. At approximately 13% nanotube loading, thebulk resistivity approached 0.08 ohm-cm, which is comparable to that ofa pure CC nanotube mat. At nanotube loadings higher than 13% (i.e.,13-25%) the PVDF composite had bulk resistivities within the ranges ofthose of pure CC nanotube mats.

Example II

[0070] Using the procedure of Example I and Kynar 761 PVDF, a comparisonwas made of the resistivity of composites prepared with Hyperion CCnanotubes and with Hyperion BN nanotubes, respectively. The results arereported in Table 2 below: TABLE 2 CC Nanotubes BN Nanotubes TensileTensile Nanotubes Resistivity Strength Resistivity Strength Batch # (%)(ohm-cm) (psi) (ohm-cm) (psi) 1 1 19.17 8145 12987 — 2 5 0.4242 67394.1081 6663 3 7 0.2825 6770 1.6136 7227 4 11 0.1290 7144 0.4664 7201

[0071] The results of Example II confirm that at low nanotube loadings,PVDF/CC nanotube composites have significantly lower bulk resistivitythan PVDF/BN nanotube composites.

Example III

[0072] Example I was repeated, except that an ultrasound sonicator or ahomogenizer was used instead of a Waring blender to disperse thenanotubes in the polymer solution. The results are reported below inTable 3: TABLE 3 Nanotubes Thickness Dispersion Resistivity Batch # (%)(inch) Method (ohm-cm) 1 1.05 0.0058 Sonicator 11.7122 2 3.03 0.0080Sonicator 0.6144 3 13.04 0.0130 Sonicator 0.0924 0.0110 Sonicator 0.09594 13.04 0.0100 Homogenizer 0.1168 0.0110 Homogenizer 0.1028 5 20.000.0090 Homogenizer 0.0381 0.0090 Homogenizer 0.0509

[0073] These results revealed that PVDF composites with low carbonnanotube loadings made with a sonicator generally had lower bulkresistivities than composites which were made with a Waring blender.PVDF composites made with a homogenizer had similar bulk resistivityvalues to those made with a Waring blender.

Example IV

[0074] Example I was repeated, except that the nanotubes were heattreated under hydrogen, argon, or air before they were dispersed intothe polymer solution. Heat treatment of the nanotubes was carried out byheating the nanotubes under a flowing gas at the following conditions:hydrogen—600° C. for 30 minutes; argon—1000° C. for 30 minutes. Airoxidation was carried out by heating the nanotubes in an oven in air at450° C. for 2 hours. The results are reported below in Table 4: TABLE 4Original Nanotubes from H₂ Treated Ar Treated Air Oxidized Example1Nanotubes Nanotubes Nanotubes Tensile Tensile Tensile R Tensile RStrength R Strength R Strength (ohm- Strength Fibril % (ohm-cm) (psi)(ohm-cm) (psi) (ohm-cm) (psi) cm) (psi) 1.09 19.1701 8145 11.6651 7456167.0608 7404 609.9 6995 5.03 0.4242 6739 0.4751 6578 0.7111 6648 2.67407474 20.00 0.0422 5318 0.0514 5456 0.0783 7855 0.1942 7051 25.00 0.04571919 0.0327 2721 — — — —

[0075] Generally, heat treatment of nanotubes under hydrogen or argondid not improve the conductivity of the PVDF composites as compared tocomposites formed from nontreated nanotubes. PVDF composites formed fromair oxidized nanotubes showed significantly poorer conductivity (i.e.,higher bulk resistivity) but higher tensile strength (at 5-20% fibrilloading) compared to composites formed from nontreated nanotubes.

Example V

[0076] Polyvinylidene fluoride-hexafluoropropylene (i.e., PVDF/HFP)copolymer was obtained from Solvay Advanced Polymers (21508) and theprocedure of Example I was repeated with the PVDF/HFP copolymer insteadof the pure PVDF Kynar 761 polymer. The results are reproduced in Table5: t,0211 TABLE 5 PVDF/ Tensile Nanotubes HFP Nanotubes ThicknessVoltage Current Resistivity Strength Batch # (g) (g) (%) (inch) (v)(amp) (ohm-cm) (psi) 1 0.024 20 0.12 2 0.12 20 0.60 0.0095 0.4250 0.00146.4563 2793 0.0095 0.2750 0.001 30.0599 0.0075 0.4150 0.001 35.81300.0080 0.6000 0.001 55.2298 0.0085 0.5200 0.001 50.8574 0.0110 0.35000.001 44.2989 3 0.22 20 1.09 0.0055 0.1025 0.001 6.4866 3296 0.00600.0700 0.001 4.8326 0.0090 0.0700 0.001 7.2489 4 0.64 20 3.10 0.00900.0075 0.001 0.7767 3343 0.0022 0.0380 0.001 0.9619 0.0110 0.0065 0.0010.8227 5 1.08 20 5.12 0.0102 0.0048 0.001 0.5633 3206 0.0082 0.00520.001 0.4906 0.0050 0.0110 0.001 0.6328 0.0060 0.0095 0.001 0.65590.0080 0.0067 0.001 0.6149 0.0100 0.0055 0.001 0.6328 6 2 20 9.09 0.00600.0036 0.001 0.2458 3695 0.0060 0.0034 0.001 0.2347 0.0035 0.0072 0.0010.2908 0.0095 0.0024 0.001 0.2580 0.0060 0.0035 0.001 0.2444 7 3 2013.04 0.0100 0.0012 0.001 0.1323 0.0045 0.0017 0.001 0.0880 0.00550.0022 0.001 0.1392 0.0032 0.0025 0.001 0.0920 0.0080 0.0014 0.0010.1289 8 1.52 10.1 13.08 0.0060 0.0015 0.001 0.1015 3165 0.0095 0.00080.001 0.0874 0.0100 0.0009 0.001 0.1070 0.0050 0.0021 0.001 0.1208 91.54 10.04 13.30 0.0055 0.0154 0.010 0.0975 0.0060 0.0124 0.010 0.08560.0045 0.0186 0.010 0.0963 0.0050 0.0125 0.010 0.0721 0.0050 0.01870.010 0.1076 0.0060 0.0125 0.010 0.0863 10 1.95 10 16.32 0.0060 0.00120.001 0.0828 11 2.54 10.18 19.97 0.0065 0.0177 0.010 0.1322 0.00800.0120 0.010 0.1105 12 2.6 10.04 20.57 0.0035 0.0026 0.001 0.1047

[0077] The PVDF/HFP copolymer has lower crystallinity than PVDF polymer.The results of Example V showed that as little as 0.6% nanotube loadingresulted in a bulk resistivity as low as 30 ohm-cm. The bulk resistivityof the PVDF/HFP composite continued to drop as the nanotube loading wasincreased to approximately 13%. The bulk resistivity dropped below 1ohm-cm at 3.1% nanotube loading and the lowest reported bulk resistivityobserved was 0.072 ohm-cm at 13.3% nanotube loading, which is within therange of bulk resistivity for a pure CC nanotube mat. However, noimprovement in bulk resistivity was observed for PVDF/HFP compositeswith more than 13.3% nanotube loading. FIG. 1 illustrates the steepnessof the drop in bulk resistivity up to 3% nanotube loading and then therather linear decrease in bulk resistivity above 3% nanotube loading. Aninset plot within the graph of FIG. 1 was provided to better displaythis linear decrease in resistivity between 3 and 13% nanotube loading.

[0078] PVDF/HFP composites with nanotube loadings up to approximately 3%appeared to have lower bulk resistivities than those of PVDF compositeswith the same nanotube loadings. Thus, at low nanotube loading, theconductivity of a PVDF composite may be improved by using a PVDF/HFPcopolymer, or a lower grade PVDF with less crystallinity, instead of apure PVDF polymer. However, it was also observed that the tensilestrength of this PVDF/HFP composite is lower and thus the selection ofthe copolymer composite or the polymer composite will depend on theproperties required in the final application.

[0079] Conversely, the bulk resistivity of the PVDF composite is lowerthan that of the PVDF/HFP composite at higher nanotube loading (i.e.,20% or greater). It was further observed that PVDF/HFP composites withover 16% nanotube loading had rough surfaces and holes, and weredifficult to mold since they broke easily.

Example VI

[0080] A different grade of PVDF/HFP copolymer (Kynar 2801) was obtainedfrom Elf Atochem and the procedure of Example I was repeated. Kynar 2801is also known as Kynar-Flex. The results are reproduced in Table 6:TABLE 6 Kynar- Nanotubes Flex Voltage Current Thickness ResistivityBatch # (g) (g) Nanotubes (%) (v) (amps) (inch) (ohm-cm) 1 0.62 20 3.01% 0.0128 0.001 0.0075 1.1046 0.0075 0.001 0.0105 0.9061 2 1.06 20 5.03% 0.0093 0.001 0.0050 0.5350 0.0064 0.001 0.0075 0.5549 3 1.5 15 9.09% 0.0035 0.001 0.0075 0.3020 0.0019 0.001 0.0104 0.2274 0.00210.001 0.0090 0.2175 0.0010 0.001 0.0200 0.2186 4 1.51 10 13.12% 0.01050.010 0.0100 0.1208 0.0245 0.010 0.0040 0.1128 0.0115 0.010 0.01100.1456 5 1.54 10.14 13.18% 0.0033 0.001 0.0040 0.1519 0.0025 0.0010.0050 0.1438 0.0125 0.010 0.0100 0.1438 6 1.5 6 20.00% 0.0006 0.0010.0080 0.0506 0.0058 0.010 0.0078 0.0521 0.0004 0.001 0.0120 0.05250.0006 0.001 0.0080 0.0552

[0081] Unlike the PVDF/HFP composite of Example V, the Kynar 2801copolymer composite exhibited lower bulk resistivity above 13% nanotubeloading. At 20% nanotube loading, the Kynar 2801 composite had bulkresistivity values of about 0.05 ohm-cm, which is within the range of apure CC nanotube mat.

Example VII

[0082] PVDF composites were prepared by melt compounding PVDF (Kynar761) and Hyperion CC nanotubes and/or graphite (Lonza KS-75) in themixing head of a Brabender mixer at 100 RPM for approximately fiveminutes at the temperature specified. Each of the mixtures were preparedby sequential addition of the compounds in the following order: PVDF,nanotubes, then graphite, unless the mixtures were premixed as indicatedby an asterisk (*). Once compounded, flat sheets were prepared bypressing small pieces of the composite between thin, chromed plates atapproximately 240° C. with the thin plates being cooled to roomtemperature in one to two minutes. Resistivity was measured with alinear four probe head. The results are reported below in Table 7: TABLE7 PVDF Nanotubes Graphite Nanotubes Temp Resistivity Batch # (g) (g) (g)(%) (° C.) (ohm-cm) 1 40 10 — 20 210   0.148* 2 42.5 7.5 — 15 215  0.171 3 51 9 — 15 230   0.117 — 245   0.126 4 49.5 10.5 — 17.5 240  0.105 5 48 12 — 20 245   0.092 6 51 9 — 12.5 250   0.156 7 58.2 1.8 —3 240  12.1 —   1.93* 8 45.5 9 14 13.1 240   0.064 9 44.8 8.2 9 13.2 240  0.075 10 60 10 5 13.3 240   0.109 11 58 10 7.5 13.2 240   0.091 12 5610 10 13.1 240   0.077   0.055* 13 52 10 12.5 13.4 240   0.068 14 63.9 011.4 0 240 1000* 15 56 0 21.25 0 240  300*

[0083] The results show that very conductive composites can be formed bymelt compounding. Premixed materials appear to yield composites withlower bulk resistivity than composites formed by sequential addition.

[0084] As shown in Batches 8-12, it was discovered that increasing thegraphite concentration in PVDF composites at a given nanotube loadingincreases the conductivity of the composite. FIG. 2, which plots theresistivity for Batch Nos. 8-12, illustrates the decrease in bulkresistivity as a function of the increase in graphite concentration inthe PVDF composite with 13% nanotube loading.

Example VIII

[0085] Resistivity tests were performed on several polymer composites atseveral nanotube loadings. The composites were made using the solutionprocedure of Example I or the melt compounding procedure of Example VII.The following polymers were used:

[0086] PVDF-Sol (PVDF/nanotube composite made from solution);

[0087] Kynar-Flex (PVDF/HFP nanotube composite made from solution);

[0088] PVDF-Melt (PVDF/nanotube composite made by melt compounding);

[0089] PPS (poly(paraphenylene sulfide)/nanotube compound made by meltcompounding);

[0090] EVA (poly(co-ethylene-vinyl acetate)/nanotube compound made bymelt compounding);

[0091] PS (polystyrene/nanotube compound made by melt compounding); and

[0092] PE (polyethylene/nanotube compound made by melt compounding).

[0093] The results of Example VIII are shown below in Table 8: TABLE 8Nanotube Nanotube (weight (volume Resistivity fraction) fraction)Polymer (ohm-cm) 0.02 0.01 PPS 19.00 0.03 0.03 Kynar-Flex 1.10 0.91 0.040.03 PPS 211.00 0.05 0.03 PPS 3.12 0.05 Kynar-Flex 0.55 0.54 0.05PVDF-Sol 0.42 0.07 0.06 PVDF-Sol 0.28 0.05 PPS 1.97 0.09 0.08 Kynar-Flex0.30 0.23 0.22 0.22 PVDF-Sol 0.23 0.11 0.10 PVDF-Sol 0.13 0.13 0.11PVDF-Melt 0.16 0.12 Kynar-Flex 0.15 0.15 0.14 0.14 0.12 0.11 0.12PVDF-Sol 0.09 0.15 0.11 PPS 0.25 0.25 0.14 PVDF-Melt 0.14 0.13 0.12 0.180.16 PVDF-Melt 0.10 0.20 0.09 EVA 0.41 0.18 PVDF-Melt 0.09 0.18Kynar-Flex 0.06 0.05 0.05 0.05 0.18 PVDF-Sol 0.04 0.09 PE 0.65 0.25 0.13PS 0.20 0.23 PVDF-Sol 0.05 0.26 0.12 PE 0.34 0.12 EVA 0.24 0.27 0.15 EVA0.25 0.13 EVA 0.23 0.14 PS 0.11 0.28 0.13 PE 0.29 0.29 0.14 PE 0.47 0.300.16 PS 0.15 0.14 EVA 0.13 0.33 0.16 EVA 0.20

[0094] The nanotube weight fraction was calculated by dividing thenanotube weight by the composite weight. The nanotube volume fractionwas calculated by dividing the volume of the nanotubes by the volume ofthe composite. These volumes were calculated by dividing each of thenanotube and polymer weights by their respective densities (the volumeof the composite is the sum of the nanotube and polymer volumes).

[0095] The results of Example VIII showed that the resistivities of thePVDF and PVDF/HFP composites are orders of magnitude lower than theresistivities of even the best conductive polymers at any given nanotubeloading level. For example, at 5% nanotube loading, the PVDF andPVDF/HFP composites had bulk resistivity values ranging from 0.42 to0.55 ohm-cm, while the bulk resistivity of the PPS composite was 3.12ohm-cm. At 20% nanotube loading, The PVDF and PVDF/HFP composites hadbulk resistivity values between 0.04-0.09 ohm-cm, which is within therange of a pure CC nanotube mat. No other polymer composite at 20%nanotube loading or at any higher nanotube loading level had a bulkresistivity value within the range of that of a pure CC nanotube mat.These differences are significant for applications where high electricalconductivity is crucial.

[0096]FIG. 3 sets forth a logarithmic plot of nanotube weight vs.resistivity. A line is drawn which unequivocally distinguishes theresistivity of the PVDF composites from all other polymer composites,illustrating clearly that PVDF composites are superior to other polymercomposites in electrical conductivity.

[0097]FIG. 4 sets forth a plot of nanotube weight vs. conductivity. AsFIG. 4 confirms, PVDF composites have clearly superior conductivity thanother polymer composites known in the art. Additionally, FIG. 4 furthershows that, unlike other polymer composites known in the art,conductivity for PVDF composites increase exponentially as the nanotubeloading is increased to approximately 20%. PVDF composites also obtainedthe conductivity of a pure CC nanotube mat (i.e., 12.5-50/ohm-cm).Composites made from other polymers were unable to reach theconductivity range of a pure CC nanotube mat, even as the nanotubeloading was increased beyond 30%.

Example IX

[0098] Multilayered structure comprising a first layer of a PVDFcomposite, a second layer of a thermoplastic or thermoplasticblend/composite, and an optional third adhesive layer between the firstand second layers. The second layer can be a nylon-6 (6,6, 11, or 12), anylon-clay composite known for excellent barrier and high heatdistortion properties, or a nylon blend. The adhesive layer can be aPVDF-nylon blend with relatively lower viscosity. This layered structurecan be fabricated in a sheet form, or into a container of any shape andsize, or into a tubing/pipe. The inner layer for the container andtubing forms is preferably a PVDF composite layer. Since the PVDFpolymer is known for its resistance to heat and hydrocarbons, and thenylon material, in particular, nylon-clay composite is known for itshigh heat distortion temperature, excellent barrier properties and goodmechanical properties, containers and tubing formed from thismultilayered structure can be used for safe storage and transportwide-range of hydrocarbons.

[0099] A multilayered tubing may be prepared using the followingmaterials: PVDF composite of Kynar 761 and 13% loading nanotube,clay-nylon-6 composite and a PVDF (Kynar 741)-Nylon-6(30%) blend. Thepreparation of a three-layer tubing was carried out in a coextrusionsystem equipped with three extruders. The inner diameter of the tubingand thickness of each layer are: inner diameter: 20 mm; thickness ofinner PVDF composite layer: 0.6 mm; thickness of adhesive PVDF-nylonblend layer: 0.2 mm; thickness of outer clay-nylon composite layer: 1mm.

Example X

[0100] A PVDF emulsion, Kynar 720, was obtained from Elf Atochem.Hyperion CC carbon nanotubes were added and dispersed into the Kynar 720emulsion using a high shear blender (i.e., Waring blender). Thewater/liquid sent in this mixture was removed by evaporation and thinspecimens of the PVDF composition with dimensions of approximately0.5′×3″×0.01′ were prepared by hot press. The resistivities weremeasured and reported below: Nanotubes Resistivity (%) (Ohm-cm) 3 3.1113 0.14 20 0.063

[0101] The results of Example X confirm that PVDF composites with lowbulk resistivity may be prepared using PVDF emulsions. At 20% nanotubeloading, the PVDF composite prepared using this method had a bulkresistivity within the range of a pure CC nanotube mat.

We claim:
 1. A method for preparing an electrically conductive compositecomprising the steps of: (a) mixing carbon nanotubes with a polymeremulsion, said emulsion comprising a liquid and a polymer selected fromthe group consisting of polyvinylidene fluoride and copolymer ofvinylidene fluoride and another monomer; and (b) removing said liquid toform a composite comprising said nanotubes and said polymer.
 2. Themethod of claim 1, wherein the liquid is water.
 3. The method of claim1, wherein said removing step is performed by evaporating said liquid.4. The method of claim 1, wherein said mixing step is performed with ahigh shear blender.
 5. The method of claim 1, wherein said mixing stepis performed with a Waring blender.
 6. The method of claim 1, whereinsaid monomer is selected from the group consisting ofhexafluoropropylene, polystyrene, polypropylene,chlorotrifluoroethylene, tetrafluoroethylene, terpolymers or olefins. 7.An electrically conductive composite made by the method of claim 1.